Internal combustion engine for a snowmobile

ABSTRACT

A snowmobile includes an improved internal combustion engine. The engine comprises a crankcase, a crankshaft rotatably journaled within the crankcase, a cylinder block assembly extending from the crankcase, and a cylinder head assembly connected to an end of the cylinder block opposite the crankcase. A cam drive mechanism including at least one camshaft is rotatably journaled within the cylinder head assembly. A transmission is coupled to the crankshaft to transmit power from the engine to the drive assembly. The cam drive mechanism is connected to the crankshaft at a first end portion of the crankshaft. The transmission is connected to the crankshaft at a second end portion of the crankshaft opposite the first end portion.

RELATED APPLICATIONS

This application is related to Japanese Patent Application No.11-184466, filed Jun. 29, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to internal combustion enginesfor land vehicles and, more particularly, to internal combustion enginesfor powering snowmobiles.

2. Description of Related Art

Snowmobiles are popular land vehicles that are operated primarily in thewinter over snowy terrain. A typical snowmobile comprises a frameassembly, a drive assembly that is coupled to the frame assembly andengages a ground surface to propel the snowmobile, an internalcombustion engine supported by the frame for powering the snowmobile,and a transmission for transmitting power from the engine to the driveassembly.

In the past, two-stroke engines were typically used to powersnowmobiles. More recently, however, four-cycle engines have been used,primarily for their reduced emissions. The four-stroke snowmobile enginetypically comprises a crankcase assembly, a crankshaft supported forrotation within the crankcase assembly, a cylinder block assembly thatextends from the crankcase, and a cylinder head assembly connected to anend of the cylinder block opposite the crankcase. A crankshaft istypically supported for rotation within the cylinder head to actuate avalve mechanism of the engine.

The crankshaft typically includes a drive sprocket located at one end ofthe crankshaft. The crankshaft may include a driven sprocket. Thecamshaft is driven from the crankshaft by a timing chain or belt thatextends around the drive sprocket and the driven sprockets.

The transmission of the snowmobile is typically driven from the end ofthe crankshaft at which the drive sprocket is located. This end of thecrankshaft typically has a relatively large diameter to transmit torquefrom the engine to the transmission. Because the drive sprocket islocated at this same end of the crankshaft, the diameter of the drivesprocket is also typically relatively large.

For proper actuation of the valve mechanism, the driven sprockets of thecamshafts generally must have a diameter that is twice the diameter ofthe drive sprocket. As a result, the size of the cylinder head whichtypically contains camshaft and driven sprockets must be relativelylarge. This undesirably increases the overall size of the engine.

In addition, in order to minimize engine vibrations during operation ofthe snowmobile, it is preferable that the center of gravity of theengine be located in the proximity of the cylinder (e.g., at the centralaxis of the cylinder in a single cylinder engine, or at the center ofthe group of cylinders in a multi-cylinder engine). However, because thelarge diameter end of the crankshaft extends from one side of the engineto power the transmission and the camshafts, the center of gravity ofthe engine typically is offset towards the end of the crankshaft.

A need therefore exists for a snowmobile having an improved four-cycleengine.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a snowmobile isprovided comprising a frame assembly and a drive assembly coupled to theframe assembly. The drive assembly includes a drive belt adapted tocontact a ground surface to propel the snowmobile over the groundsurface. An internal combustion engine is supported by the frameassembly. The engine comprises a crankcase, a crankshaft rotatablyjournaled within the crankcase, a cylinder block assembly extending fromthe crankcase, and a cylinder head assembly connected to an end of thecylinder block opposite the crankcase. A cam drive mechanism includingat least one camshaft is rotatably journaled within the cylinder headassembly. A transmission is coupled to the crankshaft to transmit powerfrom the engine to the drive assembly. The cam drive mechanism isconnected to the crankshaft at a first end portion of the crankshaft.The transmission is connected to the crankshaft at a second end portionof the crankshaft opposite the first end portion.

In accordance with another aspect of the present invention, a snowmobileis provided comprising a frame assembly and a drive assembly coupled tothe frame assembly. The drive assembly includes a drive belt adapted tocontact a ground surface to propel the snowmobile over the groundsurface. An internal combustion engine is supported by the frameassembly. The engine comprises a crankcase, a crankshaft rotatablyjournaled within the crankcase, and a cylinder block assembly extendingfrom the crankcase and defining a cylinder bore. A piston is positionedfor reciprocating movement in the cylinder bore. A connecting rod iscoupled to the piston and the crankshaft to transmit motiontherebetween. A cylinder head assembly is connected to an end of thecylinder block opposite the crankshaft, and a cam drive mechanismincluding at least one camshaft is rotatably journaled within thecylinder head assembly. A transmission is coupled to the crankshaft totransmit power from the engine to the drive assembly. The transmissionand the cam drive mechanism are coupled to the crankshaft on oppositesides of the connecting rod.

Further aspects, features and advantages of this invention will becomeapparent from the detailed description of the preferred embodiment whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will now be described with reference to the drawings of apreferred embodiment, which is intended to illustrate and not to limitthe invention. The drawings comprise seven figures.

FIG. 1 is a simplified side elevation view of a snowmobile configuredand arranged in accordance with certain features, aspects and advantagesof the present invention. Certain internal components have beenillustrated with hidden lines.

FIG. 2 is a top plan view of the snowmobile of FIG. 1.

FIG. 3 is an enlarged side elevation view, primarily showing an engineand a steering linkage.

FIG. 4 is an enlarged top plan view, primarily showing the engine andthe steering linkage.

FIG. 5 is another enlarged side elevation view, primarily showing alubrication system of the engine.

FIG. 6 is a cross-sectional view taken along the line 6—6 of FIG. 3.

FIG. 7 is a cross-sectional view showing a combustion chamber, intakeand exhaust ports, intake and exhaust valves and a valve drive mechanismof the engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

With reference initially to FIGS. 1-3, a snowmobile 30 configured inaccordance with certain features, aspects and advantages of the presentinvention is illustrated. Although the present invention will be shownand described in the context of the illustrated snowmobile 30, someaspects and features of the present invention also can be employed withother land vehicles in manners that will become apparent.

In general, the snowmobile 30 operates over a snowfield or terrain,indicated generally with the reference letter S in FIG. 1, whichtypically is covered with snow. The reference mark FW in the figuresindicates a forward direction in which the snowmobile 30 generallymoves. As used through this description, the terms “right” and “left”will mean at or to the respective sides in a top plan view relative tothe forward direction FW.

The illustrated snowmobile 30 generally comprises a frame assembly 32,which can include a plurality of frame members 34 (see FIG. 3). Theframe members 34 can be formed with sheet metal, metal pipes or the likeand preferably are assembled in any suitable manner to have sufficientrigidity. Two side panels 36 generally cover the sides of the frameassembly 32 in the illustrated arrangement. In addition, a cowlingmember or hood 38 covers a forward portion of the frame assembly 32.Preferably, the cowling member 38 is detachably coupled with the frameassembly 32 or pivotally hinged thereto at one end so as to pivot aboutthe hinged portion. The side panels 36 and the cowling member 38 can bemade of plastic or synthetic resin. A bottom plate 40 (see FIG. 3),which can be made of sheet metal, advantageously covers a bottom portionof the frame assembly 32. Thus, a substantially closed compartment isformed over a forward portion of the frame assembly 32 by the sidepanels 36, the cowling member 38 and the bottom plate 40.

A seat 44 can be disposed above a rear portion of the frame assembly 32.In some arrangements, the seat 44 can be positioned such that a rider 45can place her feet in front of the seat 44. In the illustratedarrangement, the seat 44 is disposed such that the rider 45 straddlesthe seat with a foot positioned on each side of the seat 44. Thus, inthe illustrated arrangement, a pair of foot rests 46 are disposed onboth sides of the seat 44. A windshield 47 extends upwardly from thecowling member 38 to protect the rider 45 from wind and/or snowimpinging upon him or her.

With reference to FIG. 2, the frame assembly 32, when provided with theside panels 36, the cowling member 38, the seat 44 and the wind shield46, generally is substantially symmetrically formed relative to animaginary center plane 48 extending generally vertically and fore to aftthrough the frame assembly 32. Due to the arrangement of the variousbody components, such as the seat 44, the cowling 38, and the sidepanels 36, for instance, the frame assembly 32 is substantiallyenclosed.

The side panels 36 and the bottom plate 40 placed in front of the seat44 together with the cowling member 38 define a generally closed cavity,as discussed above. A prime mover assembly 52 can be enclosed within thecavity. Because the cowling member 38 is detachably coupled with orpivotally hinged to the frame assembly 32, the rider 45, a mechanic or arepairman can access the prime mover asembly 52 for maintenance or thelike. The illustrated prime mover assembly 52 generally comprises aninternal combustion engine 54 and a transmission 56 which transmitspower from the engine 54 to a drive assembly or unit 58 through adriveshaft 60. In other words, the transmission converts the engineoutput to speed and torque. In the illustrated arrangement, thedriveshaft 60 is journaled on the frame assembly 32.

With reference again to FIG. 1, the drive assembly 58 depends from theframe assembly 32 and is generally disposed beneath the seat 44. Thedrive assembly 58, although somewhat schematically shown in FIG. 1,preferably includes a slide rail unit 64, a drive sprocket 66, a set ofidle shafts 68 and a corresponding set of idle sprockets 70. The sliderail unit 64 comprises a pair of slide rails which extend fore and aftalong the center plane 48. Preferably, the slide rails are spaced apartfrom one another. The respective idle shafts 68 extend generallytransversely and are journaled on the illustrated slide rail unit 64.The idle sprockets 70 preferably are suitably secured to the respectiveidle shafts 68.

The slide rail unit 64 together with the drive sprocket 66 and the idlesprockets 70 support an endless drive belt 76. More specifically, theslide rail unit 64 abuts a backside of the drive belt 76, which isopposite the side of the drive belt 76 facing the terrain S, and thedrive sprocket 66 engages with the drive belt 76 to provide rotationalmovement to the drive belt 76. The respective idle sprockets 70 contactthe drive belt 76 in known manners. With reference to FIG. 2, the drivebelt 76 has a relatively broad width and a longitudinal center line ofthe drive belt 76 is placed generally on the center plane 48. When thedrive sprocket 66 rotates, the drive belt 76 also rotates in a directionindicated by the arrows 78, 80 in FIG. 1. Because the drive belt 76 hasa sufficient contact area with the terrain S, the drive belt 76 producesa friction or traction force and the rotation of the drive belt 76propels the snowmobile along the terrain S.

The drive assembly 58 preferably is provided with at least onesuspension unit 68. The suspension units 84 suspend the slide rail units64 and damp movement of the suspension unit 84 relative to the frameassembly 32. The damping movement of the suspension units 84 properlyabsorbs shocks coming from rough surfaces of the terrain S and hence therider 45 can enjoy a comfortable ride.

In the illustrated arrangement, the snowmobile 30 also includes a pairof steering skis 88. Each ski 88 preferably comprises a ski member 90and a knuckle pin 92. The ski member 90 includes a contact area, whichtypically abuts on the terrain S during movement of the snowmobile 30.The knuckle pin 92 is coupled with the ski member 90 at a generally topcenter portion of the ski member 90 and allows the ski to pivot fore andaft such that the ski member 90 can follow rough surfaces of the terrainS.

With reference to FIG. 1, a pair of support members 94 supports therespective steering skis 88 at both sides of the frame assembly 32. Eachsupport member 94 preferably has one end 96 secured to the frameassembly 32. A sleeve 98 is formed at the other end of the supportmember 94. The sleeve 98 extends generally vertically and inclinesslightly rearwardly. Preferably, the sleeve 98 is welded at a midportion thereof to the support member 94. The sleeve 98 pivotallysupports the rod member 94 about a steering axis that extends generallyvertically. Through this mounting arrangement, the ski members 90 can besteered, i.e., their forward portions are selectively directed in theright or left direction.

In order to steer the skis 88, the snowmobile 30 includes a steeringlinkage 102 that is arranged and configured in accordance with certainfeatures, aspects and advantages of the present invention. Withreference now to FIGS. 2-5, the steering linkage 102 comprises asteering handle assembly 104 and a linkage assembly 106.

The illustrated steering handle assembly 104 comprises a handle post108, a handle bar 110 and a pair of grips 112. The handle post 108extends generally vertically but its top portion inclines slightlyrearward. The frame assembly 32 supports the handle post 108 withsupport members 114 (see FIG. 3) in a manner that provides for pivotalmovement of the handle post 108 about a steering axis. The handle bar110 is positioned atop the handle post 108 and is coupled thereto by acoupling member 116 or in any other suitable manner. The grips 112 canbe secured to both ends of the handle bar 110. Preferably, a throttlelever 118 is provided on the right hand side of the handle bar 110. Inthe illustrated arrangement, the handle post 108 defines a first linkagemember in the linkage assembly 106.

The linkage assembly 106 couples the steering handle assembly 104 withthe steering skis 88 such that the pivotal movement of the steeringhandle assembly 102 about the steering axis moves the steering skis 88in the right or left direction. The linkage assembly 106 in theillustrated arrangement includes two knuckle arms 120 (see FIG. 3), twotie rods 122 (see FIG. 4), a center arm 124, a relay rod 126 and apitman arm 128. Of course, other components also can be incorporated andsome components can be integrated into a single component.

In the illustrated arrangement, the knuckle arms 120 are mounted to therespective knuckle pins 92. The tie rods 122 then couple the knucklearms 120 with the center arm 124 which can pivot about a pivot axis 130extending generally vertically as indicated by the arrows 132 of FIG. 4.Of course, as shown in FIG. 3, generally vertically should be construedto encompass a slight incline to accommodate the angles formed by therotation axes running through the various components.

The forward end of the relay rod 126 in the illustrated arrangement ispivotally connected to a portion of the center arm 124. The connectionbetween the relay rod 126 and the center arm 124 preferably is offsetfrom the pivot axis 130 so that the center arm 124 pivots about thepivot axis 130 when the relay rod 126 is pushed or pulled. The otherend, i.e., the rear end, of the relay rod 126 is pivotally connected toone end of the pitman arm 128. The pitman arm 128 preferably is affixedto a lower portion of the handle post 106. In the illustratedarrangement, the relay rod 126 inclines such that the forward end of therelay rod 126 is positioned higher than the rear end. Such aconfiguration advantageously increases the area for the forward linkageto be positioned for operation by increasing the ground clearance atthat location. In other words, the simpler connection is mounted lowerthan the more complicated connection (i.e., that having more movingcomponents). In the illustrated arrangement, the relay rod 126 defines asecond linkage member.

Because of this arrangement, when the rider 45 turns the handle post 108with the handle bar 110, the pitman arm 128 pivots about an axis of thehandle post 108. With this movement of the pitman arm 128, the relay rod126 is pushed or pulled in an axial direction as indicated by the arrows134 of FIG. 4. The center arm 124 thus pivots about the pivot axis 130and moves the respective tie rods 122 right or left as indicated by thearrows 136. Both of the tie rods 122 then move in the correspondingright or left direction. For example, if the tie rod 122 on the righthand moves in the right direction, the other tie rod 122 moves also inthe right direction, and vice versa. The knuckle arms 120 then pivot therespective knuckle pins 92. Accordingly, the respective steering skis 88pivot in the right direction or left direction in compliance with thepivotal direction of the center arm 124.

With reference now to FIGS. 3-7, the prime mover assembly 52 is disposedwithin the substantially closed protective cavity defined by the sidepanels 36, the bottom plate 40 and the cowling member 38. The engine 54is placed generally forward of the transmission 56 within this cavity.

In the illustrated arrangement, the engine 54 operates on a four-cycleprinciple and includes a cylinder block 140, a cylinder head member 142,a cylinder head cover member 144, an upper crankcase member 146 and alower crankcase member 148. It is anticipated that some features,aspects and advantages of the present could be used with a two-stroke orrotary engine; however, the configuration of a four cycle engineparticularly benefits from most features, aspects and advantages of thepresent invention.

In the illustrated arrangement, the upper crankcase member 146 is placedunder the cylinder block 140 and the lower crankcase member 148 isplaced under the upper crankcase member 146. Both the crankcase members146, 148 are joined together at a coupling line 154 which is generallydefined by a lower surface of the upper crankcase member 146 and anupper surface of the lower crankcase member 148. In the illustratedarrangement, the coupling line 154 is inclined downwardly andrearwardly. In addition, the coupling line generally extends through atleast a portion of the crankshaft 178 and, more preferably, is alignedwith a rotational axis of the crankshaft.

With reference now to FIGS. 3 and 4, the upper crankcase member 146 ismounted to the frame members 34 alone or in combination with the lowercrankcase member 148 by a plurality of mount assemblies 158. Theillustrated mounting arrangement allows the engine 54 to be securelymounted to the frame assembly 32. Each mount assembly 158 preferablyincludes a bracket or stay 160, bolts 162 and an elastic member 164. Thebrackets 160 can be attached to the crankcase members 146, 148 directlyby the bolts 162 and can be affixed to the frame assembly 32 indirectlyvia the elastic members 164 by the bolts 162. The elastic members 164preferably are made of a rubber material to isolate vibration energyfrom the frame. Advantageously, because the engine 54 is mounted on theframe assembly 32 in this manner, most of the low grade vibrationproduced by the engine 54 are not transferred to the frame assembly 32.Although not shown, the transmission 56 preferably is coupled with theengine 54 and also can be mounted to the frame assembly 32 directly orindirectly via the engine 54. IN other words, in some arrangements, thetransmission 56 and the engine 54 are mounted to the frame assembly 32as a single unit.

With reference now to FIG. 6, the illustrated cylinder block 140 definestwo cylinder boxes 170. The cylinder bores 170 extend generallyvertically and are horizontally spaced from each other so as to standside by side. This type of engine, however, is only exemplary. Engineshaving other numbers of cylinder bores, having other cylinderarrangements, and/or operating on other combustion principles (e.g.,two-stroke crankcase combustion or rotary) all can be used with certainfeatures, aspects and advantages of the present invention.

A piston 172 can reciprocate in each cylinder bore 170. The cylinderhead member 142 is affixed to the top end of the cylinder block 140 and,together with the pistons 172 and the cylinder bores 170, defines twocombustion chambers 174.

The upper and lower crankcase members 146, 148 preferably close thelower end of the cylinder block 140. The crankcase members 146, 148together define a crankcase chamber 176. A crankcase 178 extendsgenerally horizontally within the crankcase chamber 176 so that an axis180 of the crankshaft 178 extends generally normal to the center plane48. In other words, the engine preferably is transversely mounted. Thecoupling line 154 crosses the axis 180 (see FIG. 5). A crankcase covermember 182 preferably covers a left end of the coupled upper and lowercrankcase members 146, 148 and substantially encases a set of bearings184 and a seat.

The crankcase 178 is journaled by the crankcase members 146, 148 and thecover member 182. In the illustrated arrangement, a plurality ofbearings 184, 186, 188, 190, which are positioned at the cover member182 and a left side portion 192, a middle portion 194 and a right sideportion 196 of the crankcase members 146, 148, respectively, support thecrankcase 178. The crankshaft 178 is connected to the pistons 172 byconnecting rods 198 and is rotated by the reciprocating movement of thepiston 172. In the illustrated arrangement, the crankshaft 178 isconfigured so that the pistons 172 move 360 degrees out of phaserelative to one another. That is, for example, when one of the pistons172 is in the power stroke, the other piston 172 is in the intakestroke.

A left side end 202 of the crankshaft 178 extends beyond the covermember 182, while the right side end 204 of the crankshaft 178 extendsbeyond the right portion 196. In the illustrated embodiment, animaginary vertical plane 206 extends through a center of the middleportion 194 generally parallel to the center plane 48. Another imaginaryvertical plane 208 which includes the crankshaft axis 180 crosses thevertical plane 206. The center of gravity G of the engine 54 preferablyexists generally in the line where both the vertical planes 206, 208cross each other and in generally a top area of the crankcase chamber176, as shown in FIGS. 3-6.

With reference to FIG. 6, in the illustrated arrangement, although theleft side portion 192 actually defines an end wall of the crankcasechamber 176, the right side portion 196 does not define the outer wallend of the crankcase chamber 176 and the chamber 176 expands furtherbeyond the right side portion 196. A bearing member 214, which will bedescribed shortly, substantially defines the right side end wall of thecrankcase chamber 176. A portion of the crankshaft 178 between themiddle portion 194 and the right side portion 196 is positioned almostat the center of the crankcase chamber 176 along the crankshaft axis180.

With reference again to FIG. 3, the engine 54 includes an air inductionsystem 220 through which air is introduced into the combustion chambers174. The induction system 220 preferably includes a plenum chamber 222,two air intake passages 224 and six intake ports 226 (FIG. 7). As willbe recognized, the number of intake passages and ports can vary.

The intake ports 226 are defined by the cylinder head member 142. In theillustrated arrangement, three of the intake ports 226 are assembledwith a single intake passage 224 and these intake ports 226 open into asingle combustion chamber 174. The intake ports 226 are repeatedlyopened and closed by intake valves 228. When the intake ports 226 areopened, the respective intake passages 224 communicate with theassociated combustion chambers 174.

The plenum chamber 222 generally functions as an intake silencer and/ora coordinator of air charges. The plenum chamber 222 preferably alsofunctions as an air cleaner and contains a cleaner element that removesforeign substances (i.e., dirt and dust) from the air. In theillustrated arrangement, a plenum chamber member 232 defines the plenumchamber 222 and is mounted to the frame assembly 32 in a conventionalmanner. The plenum chamber member 232 preferably has an air inletopening 234 that opens forwardly in the closed cavity. The illustratedintake passages 224 extend forwardly from the plenum chamber member 232.Each intake passage 224 is defined by an upstream intake duct 236, adownstream intake duct 238 and a carburetor 240 interposed between boththe intake ducts 236, 238. The respective ducts 236, 238 preferably aremade of elastic material such as rubber.

The carburetor 240 includes a throttle valve and a fuel measurementmechanism that measures an amount of fuel supplied to the associatedcombustion chamber 174 in proportion to an amount of air measured by thethrottle valve. The throttle valve is coupled with the throttle lever118 on the handle bar 110 by an appropriate control cable so that therider 45 can operate it. The fuel is introduced into the carburetor 240from a fuel supply tank 242 (FIG. 1), which preferably is disposedbetween the cowling member 38 and the seat 44, through a proper fuelsupply conduit.

The air in the cavity is introduced into the plenum chamber 222 throughthe air inlet opening 234 and then is introduced into the combustionchambers 174 through the respective intake passages 224 and the intakeports 226, as indicated by the arrow 244 of FIGS. 3 and 7. On the way tothe combustion chambers 174, the fuel is mixed with the air in thecarburetors 240 to form air/fuel charges that can be burned in thecombustion chambers 174. The engine 30, of course, can include a fuelinjection system (either direct or indirect) instead of, or in additionto, the carburetors 240, which are shown as one type of charge formersthat can be employed.

The engine 54 also includes an exhaust system 248 that discharges burntair/fuel charges or exhaust gases from the combustion chambers 174. Twoexhaust ports 250 are defined in the illustrated cylinder head member144 for each combustion chamber 174 and are repeatedly opened and closedby a corresponding set of exhaust valves 252. When the exhaust ports 250are opened, the combustion chambers 174 communicate with an exhaustmanifolds 254 (FIGS. 2 and 3) which collects the exhaust gases anddirects them away from the combustion chambers 174, as indicated by thearrow 256 of FIGS. 3 and 7. Preferably, the exhaust manifold 254 isconnected to the exhaust ports 250 by intermediate tubular members 258made of an elastic material, such as rubber. The exhaust manifold 254 iscoupled with an exhaust silencer 260 through an exhaust conduit 262. Theexhaust gases move to the silencer 260 from the exhaust manifold 254.The silencer 260 reduces exhaust noise to a predetermined level and thendischarges the exhaust gases to the atmosphere, i.e., out of the cavity,through an appropriate exhaust pipe. The exhaust system can be tuned inany suitable manner.

The engine 54 preferably has a valve drive mechanism 266 that comprisesan intake camshaft 268 and an exhaust camshaft 270. The camshafts 268,270 extend generally parallel to one another and are journaled withinthe cylinder head member 142, which has an appropriate bearingconstruction. Camshaft caps 272 (see FIG. 6), which also have a suitablebearing construction, fix the camshafts 268, 270 onto the cylinder headmember 142. The cylinder head cover member 144 defines a camshaftchamber 273 together with the cylinder head member 142.

Each illustrated intake valve 228 comprises an intake valve tappet 274.A bias spring 276 preferably urges each tappet 274 in a direction thatcloses the valve 228. The intake camshaft 268 that has cam lobes 278that can push the respective intake valve tappets 274 downwardly withthe rotation of the intake camshaft 268 against the urging force of thebias springs 276. The intake camshaft 268 thus actuates the intakevalves 228 with the cam lobes 278 that push the tappets 274.Accordingly, the associated intake ports 226 are opened and closedrepeatedly by rotation of the camshaft 268.

Like the intake valve 228, each illustrated exhaust valve 252 comprisesan exhaust valve tappet 282. A bias spring 284 urges each tappet 282such that the valve 252 is closed. The exhaust camshaft 270 also has camlobes 268 that can push the respective exhaust valve tappets 282downwardly against the urging force of the bias springs 284 with therotation of the exhaust camshaft 270. The exhaust camshaft 270 thusactuates the exhaust valves 252 with the rotation of the camshaft 270.Accordingly, the associated exhaust ports 250 are opened and closedrepeatedly by rotation of the camshaft 270.

In the illustrated arrangement, the valve drive mechanism 266 furtherincludes a decompression mechanism 288. This mechanism 288advantageously assists manual starting of the engine 54 (i.e., use of arecoil starter) by holding the exhaust valves 252 in the open positionbefore the engine 54 starts. By holding the exhaust valves in an openposition, the compression within the cylinder can be greatly reducedduring the compression stroke of this piston. After the engine 54starts, the mechanism 288 immediately releases the valves 252 for normaloperation.

With reference again to FIG. 6, the crankshaft 178 drives the camshaft268, 270 through a suitable cam drive mechanism 292. The crankshaft 178includes a drive sprocket 296 which, in the illustrated arrangement, islocated at the right side end 204 of the crankshaft 178 to the rightside of the connecting rods 198. The driven sprockets 294 have adiameter that is twice as great as a diameter of the drive sprocket 296.A flexible transmitter 298, such as a timing chain or belt, is woundaround the respective sprockets 294, 296. The crankshaft 178 thereforedrives the respective camshafts 268, 270. A rotational speed of thecamshafts 268, 270 is half of the rotational speed of the crankshaft 178because of the difference in the diameter of the respective sprockets294, 296. That is, the engine 54 completes one cycle comprising theintake stroke, compression stroke, power stroke and exhaust strokeduring two rotations of the crankshaft 178 and, thus, the valves areopened and closed once during the two cycles of the piston.

In the illustrated arrangement, the crankshaft 178 has a diameter at theright side end 204 thereof that is less than a diameter at the left sideend 202. The diameter of the left side end 202 of the crankshaft 178must be relatively large in order to transfer power from the engine 54to the transmission 56. The diameter of the right side end 204 of thecrankshaft 178, however, need not be as large. Because the cam drivemechanism 292 is connected to the right side end 204 of the crankshaft178 in the illustrated arrangement, the diameter of the drive sprocket296 can be reduced. Consequently, the diameter of the driven sprockets294, which must be twice the diameter of the drive sprocket 296, isreduced. The cylinder head member 142 and the cylinder head cover member144 which house the driven sprockets 294 can therefore be reduced insize, thereby reducing the overall size of the engine 54.

As indicated above, it is preferable that the center of gravity G of theengine 54 be located generally in the vertical plane 206 extendingbetween the cylinders 170 in order to reduce engine vibrations duringoperation of the snowmobile 30. Because, in the illustrated arrangement,the cam drive mechanism 292 is connected to the right side of end 204 ofthe crankshaft 178 instead of the left side end 202, a length of theleft side end 202 of the crankshaft 178 is reduced. Because the leftside end 202 of the crankshaft 178 has a larger diameter than the rightside end 204, and thus has a greater mass per unit length, reducing thelength of the left side end 202 serves to shift the center of gravity Gof the engine 54 towards the vertical plane 206. The location of the camdrive mechanism 292 at the right side end 204 of the crankshaft 178further serves to balance the engine 54.

The engine 54 further includes an ignition of fringe system that ignitesthe air/fuel charges in the combustion chambers 174 during every powerstroke. Each combustion chamber 174 is provided with a spark plug 300(see FIG. 6) which has an electrode 302 (see FIG. 7) exposed into theassociated combustion chamber 174. The ignition system makes a spark ateach electrode 302 at an appropriate ignition timing under control of anignited control device so that the air/fuel charge is properly ignited.The air/fuel charge burns and abruptly expands in a manner that pushesthe piston 172 downward. The movement of the pistons 172 rotates thecrankshaft 178. The burnt charge or exhaust gases are then dischargedthrough the exhaust system 248, which has been described above.

With reference again to FIG. 3, the engine 54 preferably includesbalancers 306, 308 disposed within the crankcase chamber 176 to balancethe synchronous movement of the piston and to provide smooth rotation ofthe crankshaft 178. The balancer 306 is journaled by the lower crankcasemember 148 and is placed forwardly of the crankcase 178, while thebalancer 308 is journaled by the upper crankcase member 146 and isplaced rearwardly of the crankcase 178. The respective balancers 306,308 are driven through gear configurations. In the illustratedarrangement, the crankshaft 178 has a gear 310 next to the left sidewall 192 of the crankcase members 146, 148. The balancers 306, 308 meshwith the gear 310 so that the crankshaft 178 rotates both the balancers306, 308. Preferably, the gear ratio is one-to-one to providesynchronous movement of the balancers and the crankshaft.

With reference again to FIG. 6, the engine 54 further comprises aflywheel magneto assembly 314 positioned at a location beyond thebearing member 214. The flywheel magneto assembly 314 preferably ishoused in its own chamber and includes a rotor 316 and has a shaft 138journaled for rotation by the bearing member 214. A housing member 320is affixed to the crankcase members 146, 148 so as to enclose theflywheel magneto assembly 314 therein. A joint 322 couples the shaft 318with the right side end 204 of the crankshaft 178 adjacent to thebearing member 214. The crankshaft 178 thus rotates the shaft 218 of therotor 316 through the joint 322. The rotor 316 is configured in agenerally cup-shape and a plurality of permanent magnets is affixed toan inner surface that defines the cup-shape. The flywheel magnetoassembly 314 also includes a plurality of stator coils preferablyaffixed to a support member extending from an inner surface of thehousing member 320 toward the rotor 316. The arrangement allows themagnets to intermittently pass the coils. The flywheel magneto assembly314 thus generates AC power when the magnets in the rotor 316 rotaterelative to the stator coils. Preferably, a rectifier-regulator circuitconverts the AC power to DC power and a battery accumulates the DC powerfor usage of electrical devices of the snowmobile 30.

The rotor 316 preferably is made of metal and has sufficient weight toact as a flywheel. Because the rotor shaft 318 is separately formed fromthe crankshaft 178 and is coupled with the crankshaft 178 by the joint332, the crankshaft 178 length is advantageously shortened. This isadvantageous because production of the crankshaft becomes easier.

The engine 54 also has a starter mechanism 326 that can start the engine54. The starter mechanism 326 preferably includes a starter gear 328formed around the rotor shaft 318 and a starter motor which has a motorgear that meshes with the starter gear 328. A main switch activates thestarter motor. When the rider 45 turns on the main switch before theengine 54 has started, the starter motor rotates and the rotor shaft 318is driven by the starter motor through the combination of the motor gearand the starter gear 328. The rotor shaft 318 then rotates thecrankshaft 178 through the joint 322 and the engine 54 thus is started.

In the illustrated embodiment, the starter mechanism 326 also includes amanual starter assembly 330 disposed outside of the housing 320 and atthe outer end of the rotor shaft 318. The manual starter assembly 330preferably is a recoil starter and includes a coiled rope with a handleaffixed to an outer end of the rope. By pulling the rope with thehandle, the crankshaft 178 is rotated and the engine 54 can be started.The foregoing decompression mechanism 288 can assists this manual start.The rider 45 therefore can selectively use the electrical starterassembly, which comprises the starter gear 328 and the starter motor, orthe manual starter assembly 330 for starting the engine 54.

As described above, the snowmobile 30 is provided with the transmission56, which defines the other section of the prime mover assembly 52, totransmit the output of the engine 54 to the drive assembly 58. Withreference to FIG. 3, the transmission 56 includes an automatictransmission mechanism 334, a reduction gear combination mechanism 336and a transmission shaft 338.

With reference to FIGS. 4 and 6, the automatic transmission mechanism334 preferably is generally disposed along the left side of thesnowmobile 30. The automatic transmission mechanism 334 includes a drivepulley 342 which, in the illustrated arrangement, is affixed to the leftside end 202 of the crankshaft 178. A driven pulley 344 is affixed tothe left side end of the transmission shaft 338 and a transmission belt346 is wound around both the pulleys 342, 344. The transmission belt 346conveys the output power of the engine 54 to the transmission shaft 338.

The drive pulley 342 includes a fixed member 347 and a moveable member348, which have conical shape. The moveable member 348 can move alongthe axis 180 of the crankshaft 178 and the separation between the fixedmember 347 and the moveable member 348 can vary by centrifugal force.The belt 346 thus is positioned in a valley formed between therespective members 347, 348, which have conical shapes. When the enginespeed increases, the effective diameter of the drive pulley 342 of thebelt 346 increases because the moveable member 348 moves to the right.Of course, the driven pulley size also can be varied.

As seen in FIGS. 2 and 4, the reduction gear combination mechanism 336is generally disposed on the right hand side of the snowmobile 30. Thismechanism 336 includes a gear train that has at least a relatively smalldiameter gear affixed to the transmission shaft 338 and a relativelylarge diameter gear affixed to the driveshaft 60. The gears mesh eitherdirectly or via other one or more other gears. The driveshaft 60therefore rotates in a fixed reduced speed relative to the rotation ofthe transmission shaft 338.

When the engine 54 operates under normal running condition, the outputof the engine 54 is transmitted to the transmission shaft 338 from thecrankshaft 178 through the automatic transmission mechanism 334. Thetransmission shaft 338 rotates at a speed that is defined with thevariable reduction ratio relative to the crankshaft 178 by the automatictransmission mechanism 354. The transmission shaft 338 then rotates thedriveshaft 60 in a speed that is defined with the fixed reduction ratiorelative to the transmission shaft 338 by the reduction gear combinationmechanism 336. The driveshaft 60, in turn, drives the endless drive belt76 through the drive sprocket 66. Accordingly, the drive belt 76 rotatesand the snowmobile 30 can move.

With reference to FIGS. 3-6, a lubrication system 352 is provided withinthe engine 54. The lubrication system 352 is provided for lubricatingengine portions such as bearings 186, 188, 190 and piston 172 thatrequire lubrication to avoid seizure. In the illustrated arrangement,the lubrication system 352 employs a dry-sump configuration. This typeof lubrication system 352 primarily includes a lubricant oil reservoir354, a delivery oil pump 357 and, in some arrangements, an oil returnpump 358.

With reference to FIGS. 2 and 5, the oil reservoir 354 can be disposedgenerally behind the engine 54 and can be mounted on the frame assembly32. More specifically, in the illustrated arrangement, the oil reservoir354 is positioned behind the cylinder block 140 and higher than theflywheel magneto assembly 314. The location of the oil reservoir 354 isgenerally opposite to the drive pulley 342 of the automatic transmissionmechanism 334 relative to the vertical plane 206. The illustrated oilreservoir 354 has a supply outlet port 355 at a bottom portion thereofand a return inlet port 356 at a side portion thereof. The oil reservoir354 preferably contains a preset level of lubricant oil. This level isgenerally kept substantially constant by oil that returns to thereservoir 354 after lubricating the engine portions. The oil is returnedthrough an oil circulation mechanism that works with the delivery andreturn pumps 357, 358 in the illustrated arrangement. Of course, the oilcan be returned under the forces of gravity in some arrangements.

The delivery pump 357 and the return pump 358 in the illustratedarrangement are generally disposed in a space defined between the rightside portion 196 of the crankcase members 146, 148 an the bearing member214. That is, the pumps 357, 358 are positioned lower than the oilreservoir 354. Any type of pumps, for example, a trotted-type and adisplacement-type, can be applied as the oil pumps 357, 358.

With reference to FIG. 5, in the illustrated arrangement, the crankshaft178, the return pump 358 and the delivery pump 357 have gears 362, 364,366, respectively. The gear 362 of the crankshaft 178 meshes with thegear 364 of the return pump 358 and this gear 364 meshes with the gear366 of the delivery pump 357. Such a gear train or gear combination isonly exemplary and can be of course changeable to any suitablearrangements. In addition, the pumps can be electrically driven, driveby chain or belt or any other suitable drive mechanism.

Preferably, an oil pan 370 depends from the lower crankshaft chamber 148so that the oil that has lubricated the engine portions temporallyaccumulation therein. The oil pan 370 communicates with the crankcasechamber 176 through a plurality of oil return passageways 372 (see FIG.6). The oil pan 370 also comprises an inner oil supply passage 374 (seeFIG. 5) and an oil delivery passage 376, at least in part. Both of thepassages 374, 376 communicate with the oil delivery pump 357. Anexternal oil supply conduit 378 couples the oil supply outlet port 355with the inner oil supply passage 374. The oil delivery pump 357 takesthe oil in through the oil supply passages 378, 374 and moves the oilthrough the oil delivery passage 376 as indicated with the arrows 379,380, 382 of FIG. 5. The pressurized oil is delivered to, for example,the bearings 186, 188, 190 and further to other engine portions. An oilfilter assembly 384 (see FIG. 5) preferably is provided for removingalien substances in the oil.

As noted above, the oil that has lubricated the engine portions returnto the oil pan 370 through the oil return passageways 372. Theillustrated oil pan 370 preferably has a bulge portion 388 that definesa temporary oil chamber 390 wherein the returned oil temporarilyaccumulates. With reference to FIG. 6, the bulge portion 388advantageously is formed at the bottom area of the lower crankcasemember 148 so as to be positioned generally at the center thereof alongthe axis 180 of the crankshaft 178. In other words, the bulge portion388 is positioned adjacent to the vertical plane 206 along thecrankshaft axis 180. An oil strainer 392 depends from the bottom surfaceof the lower crankcase member 148 into the temporary oil chamber 390 anda portion of the oil passes through the oil strainer 392. The oilstrainer 392 removed foreign substances from the returned oil to reducethe amount of foreign particulate matter that passes along thecirculation system beyond the strainer 392. It should be noted that thebulge portion 388 preferably is closely sized and configured toaccommodate the strainer 392 such that the protrusion of the bulgeportion 388 into the clearance area below the engine can be reduced.

The oil pump 388 is positioned along the oil return passage 396 whichconnects the temporary oil chamber 390 with the oil reservoir 354. Morespecifically, the oil return passage 396 preferably is defined betweenan inlet opening or suction port of the strainer 392 and the returninlet port 356 of the oil reservoir 354. In the illustrated arrangement,an oil cooler 398 is interposed between the oil return pump 358 and theoil reservoir 354 in the oil return passage 396. The oil cooler 398cools the oil before returning to the oil reservoir 354 because the oilthat has lubricated the engine portions accumulates much heat and itsviscosity therefore is lowered. The oil cooler 398 restores at least aportion of the lost viscosity and somewhat reconditions the oil. The oilreturn pump 358 collects the oil in the oil chamber 390 through the oilstrainer 392 as indicated by the arrow 399 of FIGS. 5 and 6 and moves itthrough the oil return passage 396 up to the oil reservoir 354 asindicated by the arrows 400, 402, 404 of FIG. 5. On the way to thereservoir 354, the oil cooler 398 removes the heat accumulated in theoil.

When the engine 54 operates, the crankshaft 178 drives the oil deliverypump 357 and the oil return pump 358 through the gear train. The oil inthe oil reservoir 354 pulled into the delivery pump 357 through theexternal oil supply passage 378 and the inner oil supply passage 374.The oil then is pressurized by the delivery pump 357 and is delivered tothe engine portions including the bearings 186, 188, 190 through the oildelivery passages 376. After lubricating the engine portions, the oildrops down to the crankcase chamber 176 and gathers in the oil chamber390 through the return passageways 372. Then the oil is pumped up by theoil return pump 358 through the oil strainer 392 and returns to the oilreservoir 354 through the oil cooler 398 due to pressurized by thereturn pump 358.

Preferably, the return pump 358 has a size larger than the delivery pump357. This is advantageous because the oil in the oil chamber 390 can bemore quickly returned to the oil reservoir 354 and the oil does notoverflow the chamber 390. The size of the oil pan 370 therefore can bereduced.

With reference to FIG. 6, in the illustrated arrangement the foreignhousing member 320 has a lower portion or second bulge portion 406projecting downward and its bottom surface is positioned slightly higherthan the bottom surface of the bulge portion 388. A space 408 is definedbetween the lower portion of the housing member 320 and the bulgeportion 388 of the oil pan 370.

As noted above, the bulge portion 388 is formed at the bottom area ofthe lower crankcase member 148 so as to be positioned generally at thecenter thereof along the axis 180 of the crankshaft 178. Thisconstruction is advantageous because all of the oil, which dropsdownward under gravity, can travel to the oil chamber 176 over generallyequal distances from all locations within the crankcase. Accordingly,oil is less likely to pool or stand and most all of the oil returns tothe oil chamber 176 over time.

It is anticipated that the lubrication system 352 can employ a wet-sumpmethod instead of the dry-sump method. In this method, the engine 54needs no oil reservoir but requires an oil pan that is relatively largerbecause the oil for circulation is stored in the oil pan. Whether thelubrication system 352 employs the dry-sump method or the wet-sumpmethod, a relatively voluminous pan generally is formed under thecrankcase chamber 176. As described above, the snowmobile 30 has alinkage assembly 106 that includes the relay rod 126 coupling thecombination of the handle post 108 and the pitman arm 128 located to therear of the engine 54 with the combination of the tie rods 122 and thecenter arm 124 located forward of the engine 54. The relay rod 126 thusmust pass through the engine area and can result in the oil pan 370being improperly formed.

In the illustrated arrangement, the relay rod 126 and the oil pan 370are generally horizontally juxtaposed with each other. In other words,the relay rod 126 extends through a region that includes the oil pan 370at approximately the same vertical height as a portion of the oil pan370 without extending through the oil pan 370. With reference to FIG. 6,the relay rod 126 preferably is positioned next to the bulge portion 388which projects downward from the oil pan 370. That is, the relay rod 126extends in the space 408 that is defined between the lower portion ofthe housing member 320 and the bulge portion 388 of the oil pan 370.Preferably, a mid portion of the relay rod 126 is generally positionedhigher than a bottom surface 410 of the bulge portion 388 and ispositioned generally at the same height as the bottom of the housingmember 320. A higher position of the relay rod 126 than the housingmember 320 is of course possible. In addition, positioning the relay rodbelow a portion of the housing member 320 but at least level with (orhigher than) the lowest portion of the engine, which may or not be thebottom surface 410 of the bulge portion 388 of the oil pan 370.

Because of this arrangement, the relay rod 126 and the oil pan 370 cancoexist without interfering with each other. In other words, the relayrod 126 can be spaced apart from the terrain S sufficiently and theengine 54 can be provided with the oil pan 370 that has a sufficientcapacity.

The arrangement also has additional advantages. One of these additionaladvantages is that the bulge portion 388 can offer some degree ofprotection for the recessed relay rod 126. For instance, in the eventthat the bottom plate 40 of the snowmobile 30 is deformed toward therelay rod 126 due to a collision with an obstruction in the terrain S,the deformed bottom plate 40 could ultimately contact and harm the rod126. In general, a rod member can be most easily damaged at is midportion when external force is exerted thereon. Because the bottomsurface 410 of the bulge portion 370 is generally positioned lower thanthe mid portion of the relay rod 126 in the illustrated arrangement, adeformed plate 40 would not likely contact the mid portion of the rod126. Thus, even if the plate were bent or otherwise distorted, the relayrod 126 would be substantially shielded from harm.

Moreover, in the illustrated arrangement, as described above, thelocation of the oil reservoir 354 is generally opposite to the drivenpulley 342 of the automatic transmission mechanism 334 relative to thevertical plane 206. This arrangement is useful for substantially equalallotment of the component weight to both sides of the snowmobile 30.

Although the present invention has been described in terms of a certainpreferred embodiment, other embodiments apparent to those of ordinaryskill in the art also are within the scope of this invention. Thus,various changes and modifications may be made without departing from thespirit and scope of the invention. For instance, various components maybe repositioned as desired. Moreover, not all of the features, aspectsand advantages are necessarily required to practice the presentinvention. Accordingly, the scope of the present invention is intendedto be defined only by the claims that follow.

1. A snowmobile comprising a frame assembly, a drive assembly coupled tothe frame assembly and including a drive belt adapted to contact aground surface to propel the snowmobile over the ground surface, aninternal combustion engine supported by the frame assembly, the enginecomprising a crankcase, a crankshaft rotatably journaled within thecrankcase, a cylinder block assembly extending from the crankcase, acylinder head assembly connected to an end of the cylinder blockopposite the crankcase, and a cam drive mechanism including at lest onecamshaft rotatably journaled within the cylinder head assembly, and atransmission coupled to the crankshaft to transmit power from the engineto the drive assembly, wherein the cam drive mechanism is connected tothe crankshaft, at a first end portion of the crankshaft, and thetransmission is connected to the crankshaft at a second end portion ofthe crankshaft opposite the first end portion.
 2. The snowmobile ofclaim 1 4, wherein the crankshaft includes a drive sprocket at thesecond end portion thereof, the camshaft includes a driven sprocket, anda flexible transmitter extends around the drive sprocket and the drivensprocket to drive the camshaft from the crankshaft.
 3. The snowmobile ofclaim 2, wherein a diameter of the driven sprocket is twice a diameterof the drive sprocket.
 4. The snowmobile of claim 1, A snowmobilecomprising a frame assembly, a drive assembly coupled to the frameassembly and including a drive belt adapted to contact a ground surfaceto propel the snowmobile over the ground surface, an internal combustionengine supported by the frame assembly, the engine comprising acrankcase, a crankshaft rotatably journaled within the crankcase, acylinder block assembly extending from the crankcase, a cylinder headassembly connected to an end of the cylinder block opposite thecrankcase, and a cam drive mechanism including at least one camshaftrotatably journaled within the cylinder head assembly, and atransmission coupled to the crankshaft to transmit power from the engineto the drive assembly, wherein the transmission is connected to thecrankshaft at a first end portion of the crankshaft, and the cam drivemechanism is connected to the crankshaft at a second end portion of thecrankshaft opposite the first end portion, wherein a diameter of thecrankshaft at the second end portion is less than a diameter of thecrankshaft at the first end portion.
 5. The snowmobile of claim 1, Asnowmobile comprising a frame assembly, a drive assembly coupled to theframe assembly and including a drive belt adapted to contact a groundsurface to propel the snowmobile over the ground surface, an internalcombustion engine supported by the frame assembly, the engine comprisinga crankcase, a crankshaft rotatably journaled within the crankcase, acylinder block assembly extending from the crankcase, a cylinder headassembly connected to an end of the cylinder block opposite thecrankcase, and a cam drive mechanism including at least one camshaftrotatably journaled within the cylinder head assembly, and atransmission coupled to the crankshaft to transmit power from the engineto the drive assembly, wherein the transmission is connected to thecrankshaft at a first end portion of the crankshaft, and the cam drivemechanism is connected to the crankshaft at a second end portion of thecrankshaft opposite the first end portion, further comprising a flywheelmagneto assembly coupled to the crankshaft at the second end portionthereof.
 6. The snowmobile of claim 5, wherein the flywheel magnetoassembly comprises a rotor having a shaft, and an end of the shaft iscoupled to the crankshaft at the second end portion thereof.
 7. Thesnowmobile comprising a frame assembly, a drive assembly coupled to theframe assembly and including a drive belt adapted to contact a groundsurface to propel the snowmobile over the ground surface, an internalcombustion engine supported by the frame assembly, the engine comprisinga crankcase, a crankshaft rotatably journaled within the crankcase, acylinder block assembly extending from the crankcase and defining atleast one cylinder bore, a piston positioned for reciprocating movementin the cylinder bore, a connecting rod coupled to the piston and to thecrankshaft to transmit motion therebetween, a cylinder head assemblyconnected to an end of the cylinder block opposite the crankcase, and acam drive mechanism including at least one camshaft rotatably journaledwithin the cylinder head assembly, and a transmission coupled to thecrankshaft to transmit power form the engine to the drive assembly,wherein the transmission and the cam drive mechanism are coupled to thecrankshaft on opposite sides of the connecting rod.
 8. The snowmobile ofclaim 7 10, wherein the crankshaft includes a drive sprocket at an endportion thereof, the camshaft includes a driven sprocket, and a flexibletransmitter extends around the drive sprocket and the driven sprocket todrive the camshaft from the crankshaft.
 9. The snowmobile of claim 8,wherein a diameter of the driven sprocket is twice a diameter of thedrive sprocket.
 10. The snowmobile of claim 7 A snowmobile comprising aframe assembly, a drive assembly coupled to the frame assembly andincluding a drive belt adapted to contact a ground surface to propel thesnowmobile over the ground surface, an internal combustion enginesupported by the frame assembly, the engine comprising a crankcase, acrankshaft rotatably journaled within the crankcase, a cylinder blockassembly extending from the crankcase and defining at least one cylinderbore, a piston positioned for reciprocating movement in the cylinderbore, a connecting rod coupled to the piston and to the crankshaft totransmit motion therebetween, a cylinder head assembly connected to anend of the cylinder block opposite the crankcase, and a cam drivemechanism including at least one camshaft rotatably journaled within thecylinder head assembly, and a transmission coupled to the crankshaft totransmit power from the engine to the drive assembly, wherein thetransmission and the cam drive mechanism are coupled to the crankshafton opposite sides of the connecting rod, wherein a diameter of thecrankshaft at the drive sprocket is less than a diameter of crankshaftat the transmission.
 11. The snowmobile of claim 7 A snowmobilecomprising a frame assembly, a drive assembly coupled to the frameassembly and including a drive belt adapted to contact a ground surfaceto propel the snowmobile over the ground surface, an internal combustionengine supported by the frame assembly, the engine comprising acrankcase, a crankshaft rotatably journaled within the crankcase, acylinder block assembly extending from the crankcase and defining atleast one cylinder bore, a piston positioned for reciprocating movementin the cylinder bore, a connecting rod coupled to the piston and to thecrankshaft to transmit motion therebetween, a cylinder head assemblyconnected to an end of the cylinder block opposite the crankcase, and acam drive mechanism including at least one camshaft rotatably journaledwithin the cylinder head assembly, and a transmission coupled to thecrankshaft to transmit power from the engine to the drive assembly,wherein the transmission and the cam drive mechanism are coupled to thecrankshaft on opposite sides of the connecting rod, further comprising aflywheel magneto assembly coupled to the end portion of the crankshaftthe adjacent the drive sprocket.
 12. The snowmobile of claim 11, whereinthe crankshaft includes a drive sprocket at an end portion thereof, thecamshaft includes a driven sprocket, and a flexible transmitter extendsaround the drive sprocket and the driven sprocket to drive the camshaftfrom the crankshaft.
 13. The snowmobile of claim 12, wherein a diameterof the drive sprocket is twice a diameter of the drive sprocket.
 14. Thesnowmobile of claim 5, wherein the crankshaft includes a drive sprocketat the second end portion thereof, the camshaft includes a drivensprocket, and a flexible transmitter extends around the drive sprocketand the driven sprocket to drive the camshaft from the crankshaft. 15.The snowmobile of claim 14, wherein a diameter of the driven sprocket istwice a diameter of the drive sprocket.
 16. A snowmobile comprising anengine driving a drive assembly, the drive assembly mounted to a frameassembly, the drive assembly including a drive belt adapted to contact aground surface to propel the snowmobile over the ground surface, aninternal combustion engine supported by the frame assembly, the enginecomprising a center of gravity, a crankcase, a crankshaft rotatablyjournaled within the crankcase, a cylinder block assembly extending fromthe crankcase and defining at least one cylinder bore, a first valveregulating communication between a combustion gas passage and a cylinderbore and a second valve regulating communication between a combustiongas passage and a cylinder bore, a piston positioned for reciprocatingmovement in the cylinder bore, a connecting rod coupled to the pistonand to the crankshaft to transmit motion therebetween, and a cam drivemechanism including a first cam lobe for actuating said first valve, asecond cam lobe for actuating said second valve, at least one camshaftrotatably journaled within the engine and configured to actuate at leastone of said first and second cam lobes, and a transmission coupled tothe crankshaft to transmit power from the engine to the drive assembly,the transmission and the cam drive mechanism being coupled to thecrankshaft on opposite sides of the connecting rod and arranged suchthat the center of gravity is located on the plane located between saidfirst cam lobe and second cam lobe.
 17. The snowmobile of claim 16,wherein the crankshaft includes a drive sprocket at an end portionthereof, the camshaft includes a driven sprocket, and a flexibletransmitter extends around the drive sprocket and the driven sprocket todrive the camshaft from the crankshaft.
 18. The snowmobile of claim 17,wherein a diameter of the driven sprocket is twice a diameter of thedrive sprocket.
 19. The snowmobile of claim 16, wherein a diameter ofthe crankshaft at the drive sprocket is less than a diameter ofcrankshaft at the transmission.
 20. The snowmobile of claim 16, furthercomprising a flywheel magneto assembly coupled to the end portion of thecrankshaft the adjacent the drive sprocket.
 21. The snowmobile of claim16, wherein the piston comprises a first piston and the connecting rodcomprises a first connecting rod and further comprising a second pistonand a second connecting rod, the second connecting rod coupled with thesecond piston and with the crankshaft to transmit motion therebetween,whereby the center of gravity of the engine is located between the firstcylinder bore and the second cylinder bore.